In recent years, a variety of technologies have been developed for electronically controllable beam shaping, for a variety of applications, such as electrically tunable lenses for camera or flash applications. Other configurations of beam shaping devices have been proposed for lighting applications, such as general illumination and vehicle lamps. Several of these technologies for controllable beam shaping have used liquid crystals.
A first approach uses a non-uniform cell gap for the liquid crystals. Such a device may include a polarizer and flat substrate with a transparent electrode layer on the flat substrate. The switchable liquid crystals are in a gap between the flat electrode of the first substrate and an electrode layer on a second substrate. In a liquid crystal cell with a non-uniform gap, the second electrode and substrate are contoured to provide the non-uniform gap between the electrodes for the liquid crystals. The gap between the electrodes essentially has the shape of a lens, and the gap contains the liquid crystals. Voltages applied to the electrodes align the liquid crystals differently relative to the contoured surface, thereby changing index of refraction of the liquid crystal layer relative to the index of refraction of the curved substrate. The change in the refraction of the liquid crystal layer relative to that of the substrate changes the focal length of the optic and thus the shape of a beam passing through the liquid crystal optic. If one maximum or minimum voltage state places the liquid crystals in an orientation so that the index of refraction of the liquid crystal layer is approximately the same as the index of refraction of the substrate having the curved surface, the cell is substantially transparent (little or no focusing or dispersion of light passing through the lens, e.g. the focal length of the cell approaches infinity). However, as difference between the index of refraction of the liquid crystal layer and the index of refraction of the substrate having the curved surface increases, the focal length of the lens formed by the liquid crystal cell decreases. The non-uniform cell gap approach, however, has some limitations, such as small radius of curvature of the cell gap and the liquid crystal birefringence, which lead to a small tunable range of beam shaping capability. An increase in the size/curvature of the cell gap, for a larger aperture lens, requires an increase in the voltage level applied to change the state of the liquid crystals in the larger gap.
A more common alternate approach utilizes a uniform cell gap for the liquid crystals in combination with patterned electrodes that produce a non-uniform electric field on the liquid crystals to control alignment. Both transparent substrates are flat. In a typical example, the electrode layer on one substrate includes elongated electrodes crossing the cell in a first direction (e.g. like spaced apart rows), and the electrode layer on the other substrate includes elongated electrodes crossing the cell in a second direction orthogonal to the first direction (e.g. like spaced apart columns). This type of cell, typical of modal liquid crystal lens arrays, may be constructed with or without resistive layers. Control of the voltages applied to the electrodes produces a controllable non-uniform electric field in the liquid crystal layer. The non-uniform field produces variations of index of refraction of liquid crystals at different regions or locations distributed across the liquid crystal layer of the cell. Differences in index or refraction of the liquid crystals, relative to the index of refraction of the substrate, provide different amounts of light refraction at the different regions or locations across the cell aperture. Generally, if the electric field profile (for a given set of applied voltages) follows a desired lens profile, then the cell will approximate a lens shaped like the desired lens profile. The limitations of the non-uniform field approach include the need for the fabrication and need for consistent, precise alignment of the patterned electrodes in order to obtain the desired performance. Complex fabrication and alignment requirements which increase manufacturing costs and may reduce product yield. Also, the driver for such a device is more complicated than that for a non-uniform gap type of optic.